fixes a crash that can be triggered when uninstalling with privext

Fixes the following crash:

05-19 22:39:55.535  1037 24513 W WindowManager: Attempted to add application window with unknown token Token{2f841 null}.  Aborting.
05-19 22:39:55.536 10844 10844 D AndroidRuntime: Shutting down VM
05-19 22:39:55.540 10844 10844 E AndroidRuntime: FATAL EXCEPTION: main
05-19 22:39:55.540 10844 10844 E AndroidRuntime: Process: org.fdroid.fdroid.debug, PID: 10844
05-19 22:39:55.540 10844 10844 E AndroidRuntime: android.view.WindowManager$BadTokenException: Unable to add window -- token android.os.BinderProxy@d8ae31 is not valid; is your activity running?
05-19 22:39:55.540 10844 10844 E AndroidRuntime: 	at android.view.ViewRootImpl.setView(ViewRootImpl.java:891)
05-19 22:39:55.540 10844 10844 E AndroidRuntime: 	at android.view.WindowManagerGlobal.addView(WindowManagerGlobal.java:372)
05-19 22:39:55.540 10844 10844 E AndroidRuntime: 	at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:128)
05-19 22:39:55.540 10844 10844 E AndroidRuntime: 	at android.app.Dialog.show(Dialog.java:454)
05-19 22:39:55.540 10844 10844 E AndroidRuntime: 	at org.fdroid.fdroid.views.AppDetailsActivity$7.onReceive(AppDetailsActivity.java:607)
05-19 22:39:55.540 10844 10844 E AndroidRuntime: 	at android.support.v4.content.LocalBroadcastManager.executePendingBroadcasts(LocalBroadcastManager.java:311)
05-19 22:39:55.540 10844 10844 E AndroidRuntime: 	at android.support.v4.content.LocalBroadcastManager.access$000(LocalBroadcastManager.java:47)
05-19 22:39:55.540 10844 10844 E AndroidRuntime: 	at android.support.v4.content.LocalBroadcastManager$1.handleMessage(LocalBroadcastManager.java:120)
05-19 22:39:55.540 10844 10844 E AndroidRuntime: 	at android.os.Handler.dispatchMessage(Handler.java:108)
05-19 22:39:55.540 10844 10844 E AndroidRuntime: 	at android.os.Looper.loop(Looper.java:166)
05-19 22:39:55.540 10844 10844 E AndroidRuntime: 	at android.app.ActivityThread.main(ActivityThread.java:7529)
05-19 22:39:55.540 10844 10844 E AndroidRuntime: 	at java.lang.reflect.Method.invoke(Native Method)
05-19 22:39:55.540 10844 10844 E AndroidRuntime: 	at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:245)
05-19 22:39:55.540 10844 10844 E AndroidRuntime: 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:921)

The !isFinishing check was already in the installReceiver part, but
somehow missing in uninstallReceiver. There's also a reference to this
here: http://blackriver.to/2012/08/android-annoying-exception-unable-to-add-window-is-your-activity-running/

I don't understand this crash, especially as the dialouge still gets
shown after adding this check (possibly the parent activity is finishing
and then immediately restarting?). But this sems to realibly fix it.

This was happening when I installed an app, used a new settings entry to
unregister privext as a device owner (by calling it via binder/aidl) and
then trying to uninstall the app I just installed again, whithout
closing f-droid inbetween.

app/src/main/java/org/fdroid/fdroid/views/AppDetailsActivity.java

@@ -591,7 +591,7 @@ public class AppDetailsActivity extends AppCompatActivity
                     String errorMessage =
                             intent.getStringExtra(Installer.EXTRA_ERROR_MESSAGE);
 
-                    if (!TextUtils.isEmpty(errorMessage)) {
+                    if (!TextUtils.isEmpty(errorMessage) && !isFinishing()) {
                         Log.e(TAG, "uninstall aborted with errorMessage: " + errorMessage);
 
                         AlertDialog.Builder alertBuilder = new AlertDialog.Builder(AppDetailsActivity.this);